1. Field of the Invention
The present invention relates to a bridge type power converter using a self turn-off element and having improved driving and protecting characteristics.
2. Description of the Related Art
An IGBT (Insulated Gate Bipolar Transistor) is often used as a switching element of an inverter bridge of a voltage type inverter formed of transistors.
FIG. 2 shows V.sub.CE (collector-emitter voltage) vs. I.sub.C (collector current) characteristics using a drive voltage V.sub.GE (gate-emitter voltage) of the IGBT as a parameter. A characteristic feature of a transistor type power element is that collector current I.sub.C becomes substantially constant when collector-emitter voltage V.sub.CE exceeds a predetermined value. In other words, this transistor type power element has a constant-current characteristic.
The transistor type power element, however, has comparatively high voltage V.sub.CE with respect to a given constant current I.sub.C, i.e., has a high DC resistance.
A switching element of this inverter bridge can be constituted by an MCT (MOS Controlled Thyristor). In this case, a reactor for suppressing a current increase rate is connected between a DC power source and the inverter bridge, and a high-speed diode for suppressing a surge voltage generated by the reactor is connected therebetween in parallel with the reactor.
The MCT has a thyristor structure constituted by PNP and NPN transistors as shown in FIG. 4. When an ON FET is turned on by a gate signal, the NPN transistor is turned on, and the PNP transistor is turned on. Therefore, the MCT is self-held to perform an operation equivalent to that of a thyristor and exhibits a low voltage drop characteristic between the anode and cathode. When an OFF FET is turned on by the gate signal, the base-emitter path of the PNP transistor is short-circuited to turn off the PNP transistor, and the NPN transistor is turned off, thereby stopping the thyristor operation. FIG. 5 shows a circuit symbol of the MCT.
FIG. 6 shows anode current I.sub.A vs. anode-cathode voltage V.sub.A-K characteristics of the MCT. As shown in FIG. 7, almost no current flows when the MCT is OFF, and a voltage drop exhibits a low value when the MCT is turned on. In addition, unlike a transistor type element, the MCT has no constant-current characteristic and therefore can be considered as a low resistance resistor when it is turned on.
In a voltage type inverter using a transistor type element as a switching element of an inverter bridge, if terminals of a load motor are short-circuited or the motor causes a layer short-circuit, a protection operation is performed as shown in FIGS. 3A and 3B. That is, after the transistor is turned on at time t.sub.1 (FIG. 3B), an overcurrent is detected at a time t.sub.2 (FIG. 3A) to turn off a drive signal, thereby safely shutting off a failure current suppressed by a constant-current characteristic of the transistor till time t.sub.3 (FIG. 3A).
Since, however, the transistor type element has a large voltage drop in V.sub.CE for a large I.sub.C (i.e., a DC resistance is high) as shown in FIG. 2, a power loss in an inverter portion is large to result in a low efficiency.
Although voltage drop V.sub.AK of a thyristor type element such as an MCT is small even if a large I.sub.A flows, the element has no current-suppressing effect (constant-current characteristic). Therefore, a reactor is inserted in the DC circuit of an inverter to suppress a rate of increase in the magnitude of a current, and a drive signal is turned off before the current is excessively increased. This operation is shown in FIGS. 7A and 7B. Maximum current Ioff.sub.max which is the upper limit of ensuring a turn-off operation is present in the MCT, and the MCT must be turned off by this maximum current or less.
When a reactor is not used, a current obtained when the terminals of a motor are short-circuited is as indicated by a broken line in FIG. 7A. In this case, since a current rapidly rises, the MCT cannot be turned off before current Ioff.sub.max is reached.
When a reactor is inserted, the leading edge of the current is suppressed as indicated by a solid line in FIG. 7A. Therefore, if the drive signal is turned off at time t.sub.2 (FIG. 7B), the current can be set to be zero at time t.sub.3 (FIG. 7A).
Although the MCT has a high efficiency because a voltage drop is small when it is turned on, it requires a reactor and a high-speed diode for suppressing a surge voltage. In addition, since the reactor must have a large capacity so as not to be saturated even with an overcurrent, an inverter having a small or middle capacity cannot be economically designed.
Meanwhile a main circuit and a driver of a voltage type inverter generally use as an inverter bridge element an IGBT and a self-extinguishing element such as a MOSFET, a bipolar transistor, and an MCT. For example, the circuit is arranged such that a DC power output is supplied to a transistor bridge via a high-speed fuse, the DC power is converted into AC power by a switching operation of the bridge, and the converted power is supplied to a load.
The above transistor bridge constitutes a 3-phase bridge by IGBTs. Each IGBT on the positive side is driven by supplying a drive signal from an individual drive power source to the gate of the IGBT. On the other hand, since the emitters of IGBTs on the negative side are commonly connected, a gate drive signal can be supplied from a common drive power source to the gates of the IGBTs.
When a power capacity of the inverter is increased, a current of each IGBT on the negative side is increased to increase a voltage drop in the wiring commonly connecting the emitters (assuming that if an inductance of the wire is L, a voltage of Ldi/dt (where i is the current and t is the time) is generated when current i is changed). In this case, if a drive power source is commonly used, noise of L(di/dt) is applied to the gates of the other IGBTs to cause an erroneous operation. Thus, three drive power sources must be used for the elements on the negative side similar to the elements on the positive side, i.e., a total of six drive power sources must be used.
In addition, most of currently used transistors are elements called a module type in which a main electrode and a cooling surface are electrically insulated and a transistor chip in the module is connected to an electrode outside the module through a bonding wire. When a transistor connected to the positive and negative sides is broken (deteriorated) to short-circuit the positive and negative terminals of a DC power source, an overcurrent flows through the transistor to fuse the bonding wire in the module. As a result, an arc is generated to dangerously scatter the outer wall of the module type transistor. Therefore, a high-speed fuse is used to interrupt a failure current.
When the capacity of an inverter is increased, a large number of parallel module elements must be used. In this case, in order to prevent a rupture of the outer wall of the module element, a high-speed fuse is connected to the collector of each IGBT because it is practically difficult to obtain a common fuse suited to protect the individual module elements connected in parallel with each other. In addition, the fuse is connected to the collector of each IGBT because a gate drive signal of the IGBT must be commonly applied to the emitter and the gate of the IGBT.
In the circuit having the above arrangement, since at least four, and preferably, six semiconductor element drive power sources must be used in the case of a 3-phase inverter, the circuit is complicated to result in an economical disadvantage.
In addition, if an arrangement in which a fuse is connected to the collector of each IGBT is adopted, a wire length for the fuse is increased to increase an inductance (L) and an inductance of the fuse itself. As a result, a surge voltage (-Ldi/dt) to be applied to the IGBT upon turning off is increased to degrade the reliability of the IGBT element.
Especially in an element having a high switching speed, a rate (di/dt) of change in a switching current is increased to a value several times that of a conventional bipolar transistor, and a surge voltage is increased accordingly. Therefore, a conventional circuit arrangement cannot be practically adopted.
In order to absorb this surge voltage, a surge energy between the connector and the emitter of an IGBT may be clamped by a series circuit of a capacitor and a diode, and the clamped energy is then discharged via a resistor.
In this circuit, however, the number of constituting elements is increased to complicate the circuit arrangement, and a surge clamp circuit must be provided for each of IGBTs connected in parallel with each other. In addition, an energy loss is produced in the resistor through which a discharge current of the clamped energy flows, thereby decreasing an efficiency.